Chemists Carl Scheele and Joseph Priestley, working in 1772 and 1774 respectively, heated several compound gases, releasing one that optimized combustion, as shown by the brightness of a test candle flame. Chemist Antoine Lavoisier, performing similar tests, found that air is about one-fifth oxygen. Lavoisier gave oxygen its name.
Earlier scientists had found clues of oxygen's existence. Leonardo da Vinci found that respiration and combustion consume a small fraction of the air present, and in 1668, John Mayow wrote about a substance he called "nitroarial spirit" that burning and breathing consumed; he was unknowingly describing oxygen.
Scheele's experiments involved heating manganese oxide, potassium nitrate and mercury oxide, while Priestley worked solely with mercury oxide. Priestley thought the gas would be toxic, and so he set a mouse inside a jar of oxygen, expecting it to suffocate quickly, but the mouse stayed in the jar for an hour without any ill effects.
Lavoisier's experiments with oxygen also found that heating mercury oxide released a weight of gas identical to the weight that the mercury oxide solid lost in the process. When this proved consistent through the heating of other materials, he devised the law of the conservation of matter, which stated that the mass of the initial materials in a chemical reaction is the same as the mass of the products at the end.